A variety of techniques are used to interconnect packaged ICs into high density modules. Some techniques require special packages, while other techniques employ conventional packages. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits. Staktek Group, L.P. has developed numerous systems for aggregating packaged ICs in both leaded and CSP (chipscale) packages into space saving topologies.
A CSP package body typically has an array of BGA (ball grid array) contacts along a planar lower side that connect a packaged IC chip to an operating environment. The array of contacts allows a high density of connections between the CSP and an operating environment, such as, for example, a circuit board or stacked high-density circuit module.
One issue that may exist when memory CSPs are stacked is signal skew. Stacked memory CSPs typically share many signals such as address and data signals. It is beneficial for operational speed and simplicity if all common signal waveforms reach their destination simultaneously. Such simultaneous signaling may also help manage deleterious signal reflections that occur at the endpoints of signal traces. Many stacked memory modules, however, connect common signal contacts along a series of traces that carry a signal to one CSP after another, not simultaneously. Consequently, skewed signals arrive at different times at different CSPs in the same module.
Yet another issue related to connecting with circuit modules arises when ICs are arranged in stacked modules. Often the footprint of a circuit module is matched to the footprint of the bottom CSP in the module. Such a footprint may not have enough contacts for all desired input/output signal connections. This is especially true when the stacked module is a “system” module having a significant amount of signaling between ICs in the module. Further, a module may need to express a different contact footprint than the bottom CSP of the module to better meet the design needs of the system in which the module is used.
What is needed, therefore, are methods and structures for stacking circuits in thermally efficient, reliable structures that have adequate input and output connections with a flexible contact footprint capability. What is also needed are methods for interconnecting integrated circuits in a manner devised to create balanced signal interconnects and lumped impedance loads.